In general, a semiconductor laser is widely employed as the light source of an optical disk system or an optical communication system. Following improvement in performance of apparatuses constituting the system, characteristic improvement of the semiconductor laser as an elemental component is demanded. In particular, wavelength shortening and higher output of a laser beam are desired as the light source of a high-density optical disk system, and a violet semiconductor laser having a lasing wavelength of about 405 nm has recently been developed with a nitride-based material, while higher output thereof is examined.
In general, facet coating treatment of setting a light emitting surface in a cavity of a semiconductor laser device to a low reflectance while setting a light reflecting surface to a high reflectance is performed in a manufacturing process in a case of providing the semiconductor laser with higher output. In a general semiconductor laser, a dielectric film consisting of an oxide film of SiO2 or Al2O3 has generally been employed for the facet coating treatment of the cavity. In this case, however, oxygen in the dielectric film diffuses into a semiconductor layer to oxidize the semiconductor layer during oscillation of a laser beam, and a non-radiative recombination level (state where light emission energy is converted to thermal energy) is generated on the interface between the semiconductor layer and a facet coating film. Consequently, the laser beam is easily absorbed by the semiconductor layer or the facet coating film, and there is such a disadvantage that catastrophic optical damage (COD) results from abnormal heat generation of the cavity facet. Facet coating treatment with a dielectric film consisting of a nitride film having no oxygen is proposed. In general, however, stress possessed by the nitride film is larger by several times to several 10 times as compared with a case of an oxide film and hence film separation (peeling) easily takes place, and there is also a disadvantage of causing cracking or peeling on the cavity facet as a result.
Therefore, a proposal for forming a thin film of a nitride between the semiconductor layer and the facet coating film consisting of an oxide film is made in general. Such a semiconductor laser device is disclosed in Japanese Patent Laying-Open No. 2007-243023, for example.
In the aforementioned Japanese Patent Laying-Open No. 2007-243023, there is disclosed a nitride semiconductor light-emitting device in which a first coating film made of an oxide is formed on a cavity facet on a light emitting side while a second coating film made of a nitride is formed to be held between the cavity facet on the light emitting side and the first coating film. In this nitride semiconductor light-emitting device described in Japanese Patent Laying-Open No. 2007-243023, supply of oxygen from the first coating film on the outermost surface exposed to the atmosphere to a semiconductor layer (cavity facet on the light emitting side) is suppressed by the second coating film made of a nitride formed to be in contact with the cavity facet.
In the nitride semiconductor light-emitting device disclosed in the aforementioned Japanese Patent Laying-Open No. 2007-243023, however, a high-energy laser beam of a large optical density having a wavelength of about 400 nm is emitted from the cavity facet, and hence heat generation is caused in the first coating film and the second coating film through which the laser beam is transmitted. In this case, oxygen desorbs from the first coating film made of an oxide due to the heat generation on the interface between the first coating film and the second coating film. Further, oxygen in the atmosphere is transmitted through the first coating film and easily incorporated into the second coating film made of a nitride, and hence nitrogen of the second coating film and oxygen desorbing from the outside and the first coating film are substituted for each other. Thus, the interface between the first coating film and the second coating film causes alteration. The reaction rate at which oxygen desorbs from the first coating film made of an oxide is larger than the reaction rate of the substitution between nitrogen and oxygen in the second coating film. Therefore, the first coating film starts to gradually deteriorate following emission of the laser beam. Consequently, the operating current of the laser device is increased, and the cavity facet is easily broken due to further heat generation following the increase in the operating current. Particularly in a semiconductor laser provided with higher output, the aforementioned point gets remarkable as the operating time of the laser lengthens, and hence there is such a problem that the reliability of the laser device lowers.